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Tropical Storm Florence Forecast Discussion Number 38
2018-09-08 22:45:07| National Hurricane Center (Atlantic)
Issued at 500 PM AST Sat Sep 08 2018 748 WTNT41 KNHC 082045 TCDAT1 Tropical Storm Florence Discussion Number 38 NWS National Hurricane Center Miami FL AL062018 500 PM AST Sat Sep 08 2018 A NOAA P3 aircraft conducted a research mission several hours ago in Florence and measured SFMR surface winds of around 60 kt and winds up to 65 kt at a flight level of 8000 feet. Velocities of 65-70 kt at 500 meters were also measured by the Doppler radar on the plane. Based on these data, the initial wind speed is raised to 60 kt. Dropsonde data also showed that the central pressure was down to 989 mb. Florence is slowly recovering from the dry air its circulation ingested while it was under the influence of strong shear. Convection in the outer bands is relatively thin but is deeper and more persistent near the center. Now that the shear has decreased and the warm waters ahead of Florence reach deeper into the ocean, continued strengthening is anticipated. In fact, the official forecast continues to show a period of rapid intensification, now beginning 12-24 hours, with Florence reaching major hurricane intensity between 36-48 hours. One fly in the ointment is that the SHIPS diagnostics are keeping mid-level relative humidities around the cyclone around 50 percent, which isn't particularly moist, but I'm going to assume that Florence will be able to scour out the dry air within its circulation in the coming days. The HCCA model and the ICON intensity consensus support maintaining a forecast peak intensity of 125 kt in 4 days or so, so no significant changes were made from the previous forecast. Regardless of the specifics of the other models--some of which are higher and some of which are lower--Florence is expected to be a powerful major hurricane on days 3 through 5 as it moves across the southwestern Atlantic Ocean. Florence is creeping westward (270 degrees) at 4 kt, trapped between high pressure to its northeast and southwest. A different blocking ridge is expected to develop north and northeast of Bermuda over the next few days, causing Florence to accelerate toward the west-northwest and northwest between days 3-5. There have been some notable shifts in the model guidance on this cycle, with the ECMWF model swinging to the northeast closer to the GFS, and the HWRF model swinging farther south along the southern edge of the guidance envelope. Despite this change in the deterministic ECMWF run, its individual ensemble members are still showing a significant spread of solutions from just north of the Bahamas to offshore the coast of North Carolina by day 5. Because of this spread, the updated NHC track forecast largely maintains continuity and remains close to the TVCN multi-model consensus. And despite the ECMWF's shift, this track prediction remains north of the HCCA and FSSE solutions. Key Messages: 1. Florence is forecast to be a dangerous major hurricane near the southeast U.S. coast by late next week, and the risk of direct impacts continues to increase. However, given the uncertainty in track and intensity forecasts at those time ranges, it's too soon to determine the exact timing, location, and magnitude of those impacts. 2. Interests along the U.S. East Coast, particularly from north Florida through North Carolina, should closely monitor the progress of Florence, ensure they have their hurricane plan in place, and follow any advice given by local officials. 3. Large swells affecting Bermuda and portions of the U.S. East Coast will continue into next week. These swells will result in life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 08/2100Z 24.6N 54.7W 60 KT 70 MPH 12H 09/0600Z 24.6N 55.4W 70 KT 80 MPH 24H 09/1800Z 24.7N 56.6W 80 KT 90 MPH 36H 10/0600Z 25.0N 58.3W 95 KT 110 MPH 48H 10/1800Z 25.4N 60.5W 110 KT 125 MPH 72H 11/1800Z 26.9N 66.4W 120 KT 140 MPH 96H 12/1800Z 29.5N 72.5W 125 KT 145 MPH 120H 13/1800Z 32.5N 77.0W 120 KT 140 MPH $$ Forecaster Berg
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